1. Field of the Invention
This invention generally relates to the field of inspection and, more particularly, to a method of and apparatus for inspecting a glass surface for the presence or nonpresence of traces of metal or metallic oxides on the glass surface.
2. Description of the Prior Art
As is well known, flat glass is commonly produced by the float process, such as illustrated and described in U.S. Pat. No. 3,743,495, issued on July 3, 1973. As there explained, the manufacture of flat glass by the float process involves delivering molten glass at a controlled rate onto a relatively wide bath of molten metal (usually tin) and advancing it along the surface of the bath under physical and thermal conditions which (1) assure that a layer of molten glass will be established on the bath, (2) that the glass in the layer can flow laterally to develop on the surface of the metal bath a buoyant body of molten glass of stable thickness, and (3) that the buoyant body in ribbon form will be continuously advanced along the bath and sufficiently cooled as it advances to permit it to be taken unharmed out of the bath by mechanical conveying means.
In this process, traces of metal or metallic oxides form on the bottom surface of the glass ribbon as it is removed from the molten bath. However, these oxides are substantially invisible to the naked eye and do not present any deleterious effects to the end use of the glass. On the other hand, one of the advantages of float glass is that it is formed with the desired surface finish and so need not be ground and polished. However, while such glass may have excellent optical quality it has been found that the bottom surface of the glass ribbon, that is, the surface in contact with the metallic bath during formation and having the metallic oxides thereon, has a greater degree of planimetry than the top surface of the glass ribbon.
As is well known, in the manufacture of laminated glass products it is desirable to have the surfaces with the better planimetry on the outer surfaces of the end product. In such units the glass surfaces laminated to the plastic interlayer as well as irregularities thereon are in effect eliminated and the optical quality of the finished unit is determined by the outwardly facing surfaces. Thus, in a manufacturing line it is desirable to have the high quality surfaces of the glass sheets facing in a known direction. In view of the foregoing discussion, the surfaces of glass sheets may be continuously inspected for the presence or nonpresence of metallic oxides thereon for determining the better quality surface of the glass sheets. Accordingly, during the processing of glass sheets, corrective action can be immediately and/or automatically taken in the manufacturing line to insure that the better quality surfaces of the glass sheets face in one direction.